Dental restoration combining dental porcelain and improved white gold alloy

ABSTRACT

A white gold alloy consisting essentially of 50-54% gold, 27-31% palladium, 11-16% silver, about 0.05-0.25% of iridium or ruthenium, and 4.5-8% of indium and tin, provided there is at least 2% indium, no more than 4.5% tin, and further provided that if there is less than about 6% indium, the alloy must contain at least 0.25% tin. The alloy meets or at least narrowly approaches the ADA Spec. No. 5 for Type IV dental casting gold alloys, and is useful for dental appliances and for the fusing of porcelain thereto for dental restorations.

This is a division of application Ser. No. 370,496, now abandoned, filedJune 15, 1973, which is a continuation-in-part of Ser. No. 190,686 filedOct. 19, 1971, now abandoned.

This invention concerns a white gold alloy useful for dental castingpurposes. More particularly, this invention concerns a white gold alloycomposition comprised of gold, palladium and silver and criticalproportions of indium, or a combination of indium and tin, and iridiumor ruthenium. The alloy has a unique combination of physical propertieswhich make it useful in the casting of dental appliances requiring"long-span" bridges, in addition to being useful for the fusing ofporcelain thereto for dental restorations having aesthetic desirability.

Certain white gold alloys are described in the literature. For example,U.S. Pat. No. 1,283,264, Oct. 29, 1918, describes an alloy of 50% gold;15% palladium and 35% silver for use in dental, electrical and jewelryapplications. British Patent 683,004, Nov. 19, 1952, describes alloysuseful in the manufacture of spinning nozzles comprised of 20 to 65%gold, 10 to 65% palladium and 15 to 50% silver, which may contain one ormore of other metals in quantities of 0.5 to 5%, such as zinc, cadmium,magnesium, tin, copper, germanium, indium, manganese, iron, cobalt andnickel. British Patent 803,379, Oct. 22, 1958, describes a jewelry alloyconsisting of 75% gold; 16% palladium; 4.5% silver; 1% copper and 3.5%indium. The reduction of grain size in noble metals and their alloys bythe addition of iridium or ruthenium thereto is disclosed by U.S. Pat.No. 2,143,217 and by J. P. Nielsen and J. J. Tuccillo, Journal of DentalResearch, Vol. 45, No. 3, Part 2, pages 964-969, May-June, 1966. U.S.Pat. No. 3,667,936 discloses a dental alloy consisting essentially ofmore than 95 wt. % of a precious alloy, including 8-50% of palladium,3-12% indium and at least 33% of gold and silver. However, thisreference does not suggest the presently claimed combination of indium,or indium plus tin, with irridium or ruthenium, in the specific andlimited proportions necessary in the white gold alloy of this invention.

The white gold alloy embodied herein consists essentially of thecomponents (in percents by weight) of 50 to 54% gold; 27 to 31%palladium, 11 to 16% silver, 4.5 to 8% of indium and tin, provided thereis at least 2% indium and no more than 4.5% tin, and further providedthat if there is less than about 6% indium, there must be at least 0.25%tin; and from about 0.05 to about 0.25% of iridium or ruthenium. Saidminor amounts of iridium or ruthenium may be based on the total of theother metals in the alloy of 100%. These grain-refining components aredesirably added to the melt as a binary alloy with gold or silver. Anexcess of these elements will show up as undesirable white spots ofundigested iridium or ruthenium in the cast alloy.

It has been discovered that the foregoing proportions of metalconstituents in the alloy are essential and critical, in particular theamount of indium, or indium and tin combined, and the iridium orruthenium content, in order to obtain the desired physical properties.For instance, the present alloy meets, or at least narrowly approaches,the requirements of American Dental Association Specification No. 5 forType IV dental casting gold alloys. In accordance with theserequirements, the hardened alloy of this invention will have a Brinellhardness number of at least about 170 and often at least 200 or higher,an ultimate tensile strength of at least about 90,000 psi., and aminimum elongation of at least about 2 %. Thus, the alloy's highstrength and hardness qualify the alloy for ADA Type IV classificationand the important utility of being useable for long span (i.e. fiveunits and above) bridge work.

The white gold alloys embodied herein have a melting range of about2100°F. to 2300°F. The cast alloys are normally heat treated in aconventional manner by heating in a furnace to 1900°F. at a rate of75°F. to 100°F. per minute, holding at 1900°F. for about 10 minutes,removing the specimen from the furnace and cooling under a refractorycover.

The alloy of this invention, after the described heat treatment, willhave an ultimate tensile strength on the order of about 90,000 psi. andhigher, a yield strength on the order of about 70,000 psi and upwards,an elongation within the range of about 2% to 13%, preferably in therange of 7 to 13%, a Brinell hardness number of about 170 and upwards,and a crystalline grain size ranging from about 10 to 30 microns.

The following examples setting forth the physical properties ofrepresentative white gold alloys demonstrate the criticality of thepresent composition with regard to the attainment of the desiredphysical properties. All physical properties for said alloys have beenmeasured after the above-described heat treating operation.

EXAMPLE 1

A white gold alloy is prepared composed of 52% gold, 27.9% palladium,16.5% silver, 0.1% iridium, 3% indium and 0.5% tin. The alloy has amelting range of 2150°-2280°F. Ultimate tensile strength is 39,500 psi,yield strength 23,700 psi, % elongation is 11.5%, Brinell hardness No.is 88, and grain size is 33 microns. This alloy is too soft and too weakto meet the requirements of an ADA Type IV dental casting gold alloy.

EXAMPLE 2

A white gold alloy of 51% gold; 29.4% palladium; 15% silver; 0.1%iridium and 4.5% indium has an ultimate tensile strength of 61,200 psi.;a yield strength of 27,500 psi.; an elongation of 32.7% and a Brinellhardness number of 101. This alloy thus fails to meet the physicalproperty requirements for an ADA Type IV casting alloy, in constrast tothe alloys of the invention described in the following examples.

EXAMPLE 3

An alloy composed of 50.5% gold; 27.1% palladium; 16% silver; 0.1%iridium; 5.8% indium and 0.5% tin has an ultimate tensile strength of94,500 psi.; yield strength of 74,500 psi., elongation of 5.3%, aBrinell hardness number of 204, and a melting range of 2140° to 2300°F.

EXAMPLE 4

An alloy of 51% gold; 29.4% palladium; 12% silver; 0.1% iridium and 7.5%indium has an ultimate tensile strength of 101,800 psi.; a yieldstrength of 80,000 psi., an elongation of 8.5% and a Brinell hardnessnumber of 205.

EXAMPLE 5

An alloy composed of 51% gold; 29.4% palladium; 12% silver; 0.1%iridium; 4.5% tin and 3% indium has an ultimate tensile strength of100,000 psi.; a yield strength of 81,000 psi.; an elongation of 2%; anda Brinell hardness number of 225.

EXAMPLE 6

A preferred alloy of this invention is prepared having the constituency;50.9% gold; 29.5% palladium; 12% silver; 0.1% iridium and 7.5% indium.

The physical properties are an ultimate tensile strength of 101,700psi.; a yield strength of 79,800 psi., an elongation of 9.9%, and aBrinell hardness number of 205; grain size is 22 microns and meltingrange is 2150°-2335°F. When corresponding amounts of indium are replacedin further alloy preparations by 2.5% tin; 3% tin; and 4.5% tin, thegeneral good properties of the alloy are noted, however, the alloybecomes harder and less ductile with increasing tin content, theelongation being reduced to about 2% at 4.5% tin. Thus, greater amountsof tin are not advantageous since the elongation will approach 1% whichis undesirable even though good ultimate tensile strength, yieldstrength and hardness properties are retained.

EXAMPLE 7

A series of eleven alloys of this invention composed of 52.5% gold, 27%palladium, 16% silver, 2.5% indium, 2% tin, and 0.05% ruthenium show thefollowing physical properties (average of the series): ultimate tensilestrength of 95,000 psi.; yield strength of 73,600 psi.; elongation of8%; Brinell hardness of 180.

EXAMPLE 8

An alloy composed of 52% gold, 27% palladium, 16% silver, 2.5% indium,2.5% tin, and 0.05% ruthenium has an ultimate tensile strength of113,000 psi., a yield strength of 84,000 psi., an elongation of 10% anda Brinell hardness of 208.

EXAMPLE 9

An alloy composed of 51% gold, 29.5% palladium, 12% silver, 7.5% indiumand 0.05% ruthenium has an ultimate tensile strength of 101,800 psi., ayield strength of 80,000 psi., an elongation of 8.7% and a Brinellhardness of 205.

EXAMPLE 10

An alloy composed of 51% gold, 29.5% palladium, 15% silver, 4.5% indiumand 0.05% ruthenium has an ultimate tensile strength of 61,000 psi., ayield strength of 27,500 psi., an elongation of 32.7% and a Brinellhardness of 101. This alloy is deficient in acceptable physicalproperties because of insufficient indium (and tin) content.

EXAMPLE 11

An alloy composed of 52% gold, 28% palladium, 16.5% silver, 3% indium,0.5% tin and 0.05% ruthenium has an ultimate tensile strength of 39,500psi., a yield strength of 23,700 psi., an elongation of 11.5% and aBrinell hardness of 88. This alloy has inadequate physical propertiesbecause of low indium and tin content.

EXAMPLE 12

An alloy composed of 50.5% gold, 27.2% palladium, 16% silver, 5.8%indium, 0.5% tin and 0.05% ruthenium has an ultimate tensile strength of94,500 psi., a yield strength of 74,500 psi., an elongation of 5.3% anda Brinell hardness of 204.

EXAMPLE 13

This example demonstrates that the presence of iridium or ruthenium isessential to obtain the high level of physical properties exhibitedherein. A representative alloy (two specimens, Alloy No. 2) fallingwithin the scope of the present invention, containing iridium in theclaimed range is prepared together with an alloy (three specimens, AlloyNo. 1) of comparable composition, except without iridium addition. Thealloy specimens are formulated, cast, heat-treated and evaluated asdescribed earlier. The data and results are summarized asfollows:Compositions______________________________________Components,Alloy No. 1 Alloy No. 2Weight percents (no iridium) (withiridium)______________________________________Gold 51.0% 51.0%Palladium29.5% 29.5%Silver 12.0% 11.9%Indium 7.5% 7.5%Iridium 0.0%0.1%______________________________________

    Physical Properties                                                           __________________________________________________________________________                Alloy No. 1                                                                   Specimen A                                                                           Specimen B                                                                           Specimen C                                                                           Average                                      __________________________________________________________________________    Ultimate Tensile                                                              Strength, psi                                                                             79,000 84,500 81,000 81,500                                       Yield Strength                                                                (at 2% offset), psi                                                                       73,500 76,500 75,000 75,000                                       Elongation, %                                                                             2.5    3.0    2.0    2.5                                          Grain Size, microns                                                                       150    150    150    150                                          __________________________________________________________________________                Alloy No. 2                                                                   Specimen A                                                                              Specimen B Average                                      __________________________________________________________________________    Ultimate Tensile                                                              Strength, psi                                                                             97,000    106,000    101,500                                      Yield Strength                                                                (at 2% offset), psi                                                                       77,500    83,500     80,500                                       Elongation, %                                                                             8.0       9.0        8.5                                          Grain Size, microns                                                                       25        25         25                                           __________________________________________________________________________    (The Brinell Hardness numbers of all alloys are about 200 ± 5%)        

The foregoing results verify that the incorporation of a minor amount ofiridium (or ruthenium) in the subject alloys provides a markedimprovement in physical properties (i.e., 24.5 percent greater ultimatetensile strength, 7.3 percent greater yield strength, and 240 percentgreater elongation, which gives a much tougher alloy, toughness beingmeasured by the product of tensile strength and elongation). The finergrain size also reduces the tendency of the alloy to crack duringsolidification after melt-casting, and to withstand stresses encounteredin dental use.

As previously mentioned, the white gold alloy of this invention isextremely well adapted for the fusing of dental porcelain thereto in thepreparation of restorations having an esthetic veneer of porcelainbonded to a noble metals alloy base. When the conventional porcelain togold fusion techniques are employed using firing temperatures of fromabout 1700°F. to about 1950°F., the resulting bond of the dentalporcelain to the gold alloy will be stronger than the porcelain itself.Accordingly, another embodiment of this invention is a dentalconstruction (i.e., dental restoration) comprising a cast metal base ofthe alloy embodied herein and covering said metal base in the shape of atooth (or teeth) and secured thereto (i.e., baked-on thereto) fusedprocelain compatible with the metal. By "compatible" is meant that thefused procelain has a coefficient of expansion essentially matching that(and preferably slightly less than that) of the gold alloy wherebyfracture of the bond and spalling is avoided during heating and coolingcycles which the construction will encounter both during fabrication ofthe restoration in the dental laboratory and in ordinary service in thepatient's mouth. Typical fusing porcelains used in preparingbaked-on-ceramic to gold restorations and the techniques used in makingsuch constructions are described, for example, in U.S. Pat. Nos.3,052,982; 3,052,983; 2,980,998; and 2,861,010. The following exampleconcisely sets forth this technique, employing a representative alloy ofthis invention.

The first step in fabricating the restoration is to obtain an impressionof the prepared teeth, this being accomplished by the dentist who makesan impression in rubber or hydrocolloid into which a stone model iscast, thus producing a replica of the patient's mouth. A "wax-up" isdesigned from the stone model in which it is sought to restore the teethas closely as possible to the original anatomy. This wax impression orpattern of the restored tooth structure is invested in a casting ring."Investment" means that refractory material is poured around the waxpattern previously connected to a wax rod which will act as a path forthe wax to be melted and flowed out of the investment, and act duringcasting as a path for the molten metal to flow into the space previouslyoccupied by the wax pattern. The refractory material (usually aphosphate-bonded silica) is heated to approximately 1300°F., after whichit is removed from the oven and placed in a centrifugal casting machine.The casting is made by forcing molten metal (centrifugal force) into thecavity.

After the model has solidified and cooled to room temperature, theinvestment is removed from the resulting casting by sandblasting. Thecasting is then finished by removing any metallic or non-metallicprojections that are not part of the final restoration. The surface ofthe model to which porcelain is to be applied is ground to shape using aceramic bonded stone. The casting metal may then optionally be heated toa temperature of approximately 1900°F. (100°F. above the procelainfiring temperature) in order to volatize any solid contaminants from thesurface of the metal. (This heat treatment will ordinarily causeformation of a relatively dark oxide on the surface of the white goldalloy casting.) When the casting has cooled it may be immersed inhydrofluoric or hydrochloric acid to (1) remove any silica particlesthat may remain from previous operations and (2) remove the oxide formedduring the previous thermal treatment. The casting is finally cleaned ina detergent and distilled water, followed by an alcohol rinse.

A thin coat of opaque dental porcelain (as a slurry) is painted onto thesurface of the casting to be porcelainized. The casting and opaque areheated to the recommended firing temperature of the opaque, generally ina range of about 1800°-1850°F. (The purpose of the opaque is to mask outthe underlying color of the metal so that the body and incisal porcelainwhich is next applied will be its true color, rather than being changedby the background metal.)

The body and incisal porcelain is then applied to build the restorationup to its final shape. The porcelain is fired to the recommendedtemperature, generally 1700°-1800°F., and then ground to final shapewith a rotating stone. The final step is to glaze the surface of theprocelain to produce a surface impermeable to mouth fluids and foodparticles. In addition, the final restoration can be stained to matchthe adjacent tooth color in the mouth.

I claim:
 1. A dental restoration comprising a cast white gold alloymetal base in the shape of a tooth, said alloy consisting essentially,in percent by weight, of 50 to 54% gold; 27 to 31% palladium; 11 to 16%silver; about 0.05 to about 0.25% of a Group VIII metal selected fromthe group consisting of iridium and ruthenium, and a total of 4.5 to 8%indium and tin, provided there is at least 2% indium and no more than4.5% tin, and further provided that if there is less than about 6%indium, there must be at least 0.25% tin, and dental porcelain coveringsaid metal base and baked-on-thereto.
 2. A restoration according toclaim 1 in which the alloy contains 52.5% gold, 27% palladium, 16%silver, 2.5% indium and 2% tin.
 3. A restoration in accordance withclaim 2 in which the alloy contains about 0.05% ruthenium.
 4. Arestoration in accordance with claim 1 in which the alloy contains 51%gold, 29.5% palladium, 12% silver, and 7.5% indium.
 5. A restoration inaccordance with claim 4 in which the alloy contains about 0.1% iridium.6. A restoration in accordance with claim 4 in which the alloy containsabout 0.05% ruthenium.
 7. A restoration according to claim 1 in whichthe alloy contains 52% gold, 27% palladium, 16% silver, 2.5% indium and2.5% tin.
 8. A restoration in accordance with claim 7 which containsabout 0.05% ruthenium.